Although applicable to any exhaust gas sensor devices, the exemplary embodiments and/or exemplary methods of the present invention and the problems on which it is based are explained from the standpoint of use in automobiles.
Due to ever more stringent exhaust gas legislation, the limiting values for gaseous pollutants are constantly being lowered. One main problem here is that most crude emissions (HC, CO, NOx, . . . ) are generated in the cold-start phase of the engine, i.e., as long as the engine is still relatively cold. To comply with the required low limiting values, early readiness of the exhaust gas sensors, in particular the lambda sensor, is urgently needed. This is counteracted by the high risk of water shock in the sensors (thermal shock of the lambda sensor) during the cold-start phase of the engine. After the last driving cycle, water collects in the exhaust gas line, which is deposited during a renewed start on the sensors not as water vapor but rather as water droplets due to the cold exhaust gas line. Thermal shock phenomena (thermomechanical loads and stresses) occur in the ceramic sensor element because a fully ready-to-use lambda sensor has an operating temperature above 680° C. As a countermeasure, lambda sensors are heated only slowly or not at all when starting the engine (for example, with a heating time of more than 30 s) or in increments with holding ramps. However, these delayed heating strategies are used at least long enough to reach the end of the dew point at the location of the lambda sensor. The end of the dew point is the point in time after which there is no longer any condenser water or the formation of condensate of water present in the exhaust gas is overcome. When the end of the dew point is reached at the location of the lambda sensor, the lambda sensor is heated as quickly as possible to the operating temperature (approximately 680° C.) because then there is only a reduced risk of water shock (thermal shock).
Until the end of the dew point in the exhaust gas and the operating temperature of the lambda sensor of more than 680° C. have been reached, the engine is in an unregulated state in which most of the crude emissions are generated. The required high use temperature of the lambda sensor is based on the sensor mechanism. Only at a temperature above 680° C. is the oxygen ion conductivity high enough in the yttrium-stabilized zirconium oxide (electrolyte) which supplies the sensor signal (current).
Presently there are not any known cost-relevant sensor concepts or measurement strategies whereby information about the oxygen concentration present in the exhaust gas could be obtainable during the cold-start phase.
Gas-sensitive field-effect transistors based on semiconductors (ChemFETs) are being used to an increasing extent in gas sensor systems. Semiconductor materials having a wide band gap, e.g., silicon carbide (SiC) and gallium nitride (GaN), are suitable for use in exhaust gas in particular. When the gas to be detected is applied, it usually results in a change in the current (channel current) flowing from the source electrode through the transistor to the drain electrode. Such a ChemFET based on silicon carbide as the hydrocarbon gas detection device is described in U.S. Pat. No. 5,698,771.